1. Field of the Invention
The present invention relates to a liquid crystal composition containing a polymerizable compound and a liquid crystal display element manufactured using the liquid crystal composition.
2. Description of the Related Art
Polymer sustained alignment (PSA) liquid crystal display elements have a polymer structure formed in cells for controlling the pretilt angle of liquid crystal molecules. Because of their high-speed responsivity and high contrast, PSA liquid crystal display elements are being developed as liquid crystal display elements.
PSA liquid crystal display elements are manufactured by injecting a polymerizable composition, which contains a liquid crystal compound and a polymerizable compound, between substrates and polymerizing the polymerizable compound to fix the orientation of liquid crystal molecules, which have been oriented through the application of a voltage. A display defect “burn-in” of PSA liquid crystal display elements is known to be caused by impurities and variations in the orientation of liquid crystal molecules (variations in pretilt angle).
Burn-in due to impurities is caused by a polymerizable compound remaining after incomplete polymerization and a polymerization initiator for promoting polymerization. Thus, it is necessary to minimize the amount of polymerizable compound remaining after polymerization and the amount of polymerization initiator to be added. For example, the addition of a large amount of polymerization initiator so as to complete polymerization results in a decrease in the voltage holding ratio of a display element because of a residual polymerization initiator, thus adversely affecting display quality. However, an excessive decrease in the amount of polymerization initiator so as to prevent a decrease in the voltage holding ratio results in incomplete polymerization and the presence of a residual polymerizable compound, which will cause burn-in. In order to complete the polymerization of a polymerizable compound with a minimum amount of polymerization initiator to decrease the amount of residual polymerizable compound, a large amount of energy, such as strong UV irradiation for a long period of time, may be applied. However, this requires large manufacturing equipment, results in decreased manufacturing efficiency, and causes deterioration of the liquid crystal material due to UV irradiation. It is therefore difficult for existing liquid crystal compositions containing a polymerizable compound to simultaneously decrease the amounts of residual polymerizable compound and residual polymerization initiator. Thus, there is a demand for a liquid crystal composition containing a polymerizable compound that can be completely polymerized without using a polymerization initiator.
It is also known that burn-in can result from variations in the pretilt angle of liquid crystal molecules in a liquid crystal composition containing a polymerizable compound. More specifically, when a polymer produced by the polymerization of a polymerizable compound is flexible, prolonged display of a single pattern with a display element manufactured using the polymer can change the structure of the polymer and consequently change the pretilt angle. Variations in pretilt angle can cause burn-in. Thus, the polymerizable compound should form a polymer having a rigid structure.
In order to prevent burn-in by increasing the rigidity of a polymer, Japanese Unexamined Patent Application Publication No. 2003-307720 discloses a display element manufactured using a polymerizable compound having a 1,4-phenylene group, which only has a ring structure and a polymerizable functional group, and Japanese Unexamined Patent Application Publication No. 2008-116931 discloses a display element manufactured using a polymerizable compound having a biaryl structure. However, these polymerizable compounds have low compatibility with liquid crystal compounds and may be precipitated in liquid crystal compositions.
Also in order to prevent burn-in by increasing the rigidity of a polymer, Japanese Unexamined Patent Application Publication No. 2004-302096 discloses a display element manufactured using a liquid crystal composition that contains a mixture of a bifunctional polymerizable compound and an at least trifunctional polymerizable compound, such as dipentaerythritol pentaacrylate or dipentaerythritol hexaacrylate. However, dipentaerythritol pentaacrylate and dipentaerythritol hexaacrylate, which have no ring structure, have low affinity for liquid crystal compounds and are therefore less likely to control the orientation of liquid crystal molecules, resulting in insufficient orientation stability. Furthermore, these polymerizable compounds require a polymerization initiator in polymerization and can remain after polymerization without a polymerization initiator.
In the case of PSA liquid crystal display elements for use in liquid crystal television sets, there is a strong demand for PSA liquid crystal display elements having high-speed responsivity so as to improve motion picture display performance. In order to reduce response time, Japanese Unexamined Patent Application Publication No. 2009-102639 and WO 2010/084823 disclose a liquid crystal composition containing a polymerizable compound that contains a non-polymerizable liquid crystal compound having an alkenyl group. However, these patent literatures focus solely on the non-polymerizable liquid crystal compound and do not describe a combination of the non-polymerizable liquid crystal compound and the polymerizable compound. Consequently, there are some problems, including the precipitation of the polymerizable compound.
Thus, it is difficult for these techniques to satisfy characteristics required for a liquid crystal composition containing a polymerizable compound, such as high-speed responsivity, burn-in resistance of display elements, orientation stability, stability of the composition without precipitation, and high efficiency in the manufacture of PSA liquid crystal display elements. Thus, these techniques should be further improved.